Comedians have personality types linked with psychosis, like many other creative types, which might explain why they can entertain, researchers claim.

They score highly on characteristics that in extreme cases are associated with mental illness, a study by Oxford University researchers suggests.

Unusually, they have high levels of both introversion and extroversion.

The team says the creative elements needed for humour are similar to traits seen in people with psychosis.

The idea that creativity in art and science is connected with mental health problems has long captured the public imagination.

However, there has been little research on whether comedians have some of the traits - in a healthy form - associated with psychosis (delusions or hallucinations that can be present in schizophrenia and bipolar disorder).Unusually introverted

Researchers from the University of Oxford and Berkshire Healthcare NHS Foundation Trust studied 523 comedians (404 men and 119 women) from the UK, US and Australia.

The comedians were asked to complete an online questionnaire designed to measure psychotic traits in healthy people.

The questionnaire was also completed by 364 actors - another profession used to performing - as a control group, and by a group of 831 people who worked in non-creative areas.

The researchers found that comedians scored significantly higher on all four types of psychotic personality traits than the general group, with particularly high scores for both extroverted and introverted personality traits.

The actors scored higher than the general group on three types - but not on introverted the personality aspect.

The researchers believe this unusual personality structure may help explain the ability of comedians to entertain.Thinking 'outside box'

Professor Gordon Claridge, of the University of Oxford's Department of Experimental Psychology, said: "The creative elements needed to produce humour are strikingly similar to those characterising the cognitive style of people with psychosis - both schizophrenia and bipolar disorder."

He said although schizophrenic psychosis itself could be detrimental to humour, in a lesser form it could increase people's ability to associate odd or unusual things or to think "outside the box".

Manic thinking, which is found in those with bipolar disorder, may help people combine ideas to form new, original and humorous connections, he added.

Prof Claridge told BBC News: "Comedians tend to be slightly withdrawn, introverted people who may not always want to socialise, and their comedy is almost an outlet for that. It's a kind of self-medication."

Dr James MacCabe, of the Institute of Psychiatry, at King's College, London, said: "Psychosis is not a problem with personality, it's a more severe disorder than that.

"People with psychosis and schizophrenia have a very impaired ability to appreciate humorous material.

"This study tells us some interesting things about the differences between comedians and actors but not about the link with psychosis."

Paul Jenkins, CEO of the charity Rethink Mental Illness said these were interesting findings, but we must guard against the "mad creative genius stereotype".

"Mental illnesses like schizophrenia can affect anyone, whether they are creative or not. Our knowledge and understanding of mental illness still lags far behind our understanding of physical illnesses, and what we really need is much more research in this area."